Whether to enable client-side validation for this validator.
The actual client-side validation is done via the JavaScript code returned
by clientValidateAttribute(). If that method returns null, even if this property
is true, no client-side validation will be done by this validator.

A PHP callable that replaces the default implementation of isEmpty().
If not set, isEmpty() will be used to check if a value is empty. The signature
of the callable should be function ($value) which returns a boolean indicating
whether the value is empty.

The user-defined error message. It may contain the following placeholders which
will be replaced accordingly by the validator:

{attribute}: the label of the attribute being validated

{value}: the value of the attribute being validated

Note that some validators may introduce other properties for error messages used when specific
validation conditions are not met. Please refer to individual class API documentation for details
about these properties. By convention, this property represents the primary error message
used when the most important validation condition is not met.

A PHP callable whose return value determines whether this validator should be applied.
The signature of the callable should be function ($model, $attribute), where $model and $attribute
refer to the model and the attribute currently being validated. The callable should return a boolean value.

This property is mainly provided to support conditional validation on the server-side.
If this property is not set, this validator will be always applied on the server-side.

The following example will enable the validator only when the country currently selected is USA:

A JavaScript function name whose return value determines whether this validator should be applied
on the client-side. The signature of the function should be function (attribute, value), where
attribute is an object describing the attribute being validated (see clientValidateAttribute())
and value the current value of the attribute.

This property is mainly provided to support conditional validation on the client-side.
If this property is not set, this validator will be always applied on the client-side.

The following example will enable the validator only when the country currently selected is USA:

The list of attributes to be validated.
Note that if an attribute is not associated with the validator - it will be
ignored. If this parameter is null, every attribute listed in $attributes will be validated.

validateValue()
protected method

Validates a value.

A validator class can implement this method to support data validation out of the context of a data model.